1. Field of the Invention
The present invention relates to a hand-held instrument for reflection measuring on printed sheets and test charts.
2. Description of Prior Art
Hand-held instruments of the afore-described type are known, for example, from German Patent 43 05 968. Measuring of the optical density and of the colorimetrical characteristic values on printing control strips and test charts is accomplished by a movement manually performed by the instrument on the measuring surface, thereby scanning a major number of measuring fields. Moreover, the hand-held instrument is able to perform measurements within the image, with the instrument being positioned on the selected points with repetitive accuracy.
Reflection measurements on printed sheets and test charts are performed for determining the optical density and the characteristic values derived therefrom, such as dot gain and print contrasts. Moreover, colorimetrical measurements are performed by tristimulus instruments and spectral photometers for determining the CIE-values on test charts and printed products of the most various types. Some of the spectral photometers are suitable to measure both the characteristic values of the optical density and the colorimetrical characteristic values in combination.
In density measurements on prints, printing control strips are printed at the edge of the sheet, with the printing control strips including series-arranged control fields for the colors involved in multi-color prints. The preferred standard process colors for the four-color print are cyan, magenta, yellow and black. In the majority of cases, the control fields are of a width of only 5 to 6 mm so that a printing control strip extending across the entire printing width, with an average sheet format, can consist of more than 200 control fields.
Test charts used for full-format testing and for optimizing the printing quality of the printing machines contain an even larger number of control fields. A completely new type of test charts has been developed for the color calibration of digital proof instruments, the control fields of which, mostly, are colorimetrically measured. In order to achieve the color calibration computed by special software programs, substantially for the entire visible color space, test charts have been developed for the so-called color management that contain between 200 and 2000 measuring fields. The conventional test chart of the international ANSI IT 8.7/3-1993 standard contains, for example, 928 measuring fields.
Conventionally, printing control strips and test charts are evaluated by hand-held instruments suitable to successively measure respectively one control field at a time. Instruments of that type are inexpensive but too slow for a fast and comfortable evaluation of a large number of control fields. This disadvantage could not be overcome either by a design in which the measuring head is moved by motor power (see European Patent 0 171 360) according to which the instrument must still be moved from one field to the next one and a circular aperture designed as a detector is required to be precisely positioned within the measuring field before releasing the power-driven motion of the measuring head, with the latter being moved from an opening within the instrument, stopping briefly over the measuring field and then returning to its initial position.
An essential improvement has been achieved with a hand-held instrument in which the power-driven measuring head is moved along a line across a plurality of control fields, at the same time measuring the individual fields (see German Patent 37 23 701). In this way, between twelve and twenty-four fields can be scanned in one run.
An evaluation of complete printing control strips across the entire length thereof in one run at the present time can be performed only by large scan-densitometers which are true measuring machines that, technically, are highly sophisticated and correspondingly expensive.
Even higher technical efforts are involved with plotters moving the measuring head in the XY-direction thereby enabling the automatic evaluation of entire test charts. However, plotters of this type involve the unique advantage that they also enable measurements within the printed picture to be performed because the measuring head can be focused on any desired points within the picture with a high repetitive accuracy, while a hand-held instrument permits retrieval of predetermined points of measurement in the picture to a limited extent only.